The present invention is directed to a connection device for positional fixing of a gradient coil assembly in a basic field magnet assembly of a nuclear magnetic resonance tomograph. The invention is also directed to devices with components that are comparable to those cited. For example, the invention is also applicable in principle when it concerns an assembly that comprises a plurality of tube-shaped segments in a gradient coil assembly.
In particular, the invention is not limited to devices whose relevant components have the designations “gradient coil assembly” and “basic field magnet assembly”. Thus the invention also concerns as a matter of course a “magnetic resonance tomograph”, for example. In addition, the invention refers to the use of the specified connection device.
A domain of nuclear magnetic resonance tomography is the diagnosis of illnesses of the central nervous system. In particular, cerebral infarcts and the initial stages of multiple sclerosis may be detected earlier with this method than with computer tomography. Nuclear magnetic resonance tomography works without the use of ionized radiation. Consequently, despite the higher cost, nuclear magnetic resonance tomography is a standard instrument today in many large hospitals and practices.
Nuclear magnetic resonance tomography uses what is called the nuclear magnetic resonance of atomic nuclei in a magnetic field under irradiation of electromagnetic waves. The electromagnetic waves required to excite the atomic nuclei are generated by radiofrequency coils that emit pulsed waves. During the pauses between waves, the device receives the nuclear magnetic resonance emitted by the excited atomic nuclei.
In order to be able to visually display multidimensional body sections, the location of the origin of the emitted waves must be determined. In addition to the existing constant magnetic field, which is called the “basic (magnetic) field”), an additional magnetic field, which is known as the “gradient (magnetic) field”, is added that exhibits different magnitudes at every location. This gradient magnetic field is generated by the gradient coils. If waves of varying energies are radiated simultaneously, the emitted waves also thus exhibit different energies. Because each energy of the emitted waves is dependent on the magnetic field, and this magnetic field is different at every location in the body, the emitted waves can thus be associated with a specific location in the body.
During operation of a nuclear magnetic resonance tomograph, the basic field magnet assembly generates a static basic field with a magnitude of approximately 0.25 to 3 tesla (T). The gradient coil assembly usually comprises a plurality of coils in order to generate three standing magnetic field gradients perpendicular to one another. Typical values for the field strengths generated by the gradient coils are in the range of approximately 60 millitesla per meter (mT/m).
Each gradient coil is typically permeated by a pulsed current in the range of up to 300 amperes (A) with rise times of less than a millisecond (ms). Lorentz forces thereby affect the conductors of the gradient coils in the magnetic basic field, varying cyclically corresponding to the current flow through the gradient coils. These forces excite the gradient coil assembly to oscillate. The occurring forces are so great that considerable noise exposures of up to more than 100 decibels occur for a person in the nuclear magnetic resonance tomograph during the measurement.
A nuclear magnetic resonance tomograph commonly comprises a cylindrical opening that is primarily bordered by the basic field magnet assembly. A gradient coil assembly is positioned in this opening and fixed in its location. A positional fix as precise as possible plays an important role in terms of the positional resolution during the measurement with a nuclear magnetic resonance tomograph. This is again directly associated with the quality of the measurement results.
The gradient coil assembly commonly exhibits a primarily cylindrical shape. A bed for a person to be examined is located inside the gradient coil assembly.
For the reasons cited above, it is endeavored to position the gradient coil assembly as precisely as possible opposite the basic field magnet assembly and to fix it in this position.
A nuclear magnetic resonance tomograph is known from U.S. Pat. No. 6,107,799, whose disclosure is incorporated herein by reference thereto and which claims priority from DE 197 22 481 C2 and comprises a noise minimizing device to dampen the oscillations of the gradient coil assembly. In an embodiment of this invention, the gradient coil assembly is fixed opposite the basic field magnet assembly via a plurality of discrete wedges.
Due to the use of a plurality of wedges, the positioning and mounting of the gradient coil assembly is relatively elaborate and time-intensive. In addition, discrete forces develop via the use of the wedges that quasi-selectively change or effect the corresponding surface of the gradient coil assembly. This can effect the deformation of the gradient coil assembly and can be a source for measurement inaccuracies.
Alternatively according to the prior art, the gradient coil assembly is fixed opposite the basic field magnet assembly by means of a screwed connection. The previously cited disadvantage can also occur here, since the forces discretely effect the gradient coil assembly.